The default now is end of window, right? Doesn't that alleviate the problem that the original change was supposed to fix?
On Fri, Feb 12, 2021 at 12:25 AM Robert Bradshaw <rober...@google.com> wrote: > The default timestamp combiner used to be earliest as well. > > On Fri, Feb 12, 2021 at 12:10 AM Reuven Lax <re...@google.com> wrote: > >> IIRC, this was introduced because at the time users complained that >> sliding windows were virtually unusable for reasonably-sized windows. >> However this was before we allowed customizing the timestamp combiner, so >> maybe this is less of a problem now? >> >> On Thu, Feb 11, 2021 at 10:53 PM Robert Bradshaw <rober...@google.com> >> wrote: >> >>> On Wed, Feb 10, 2021 at 8:03 PM Kenneth Knowles <k...@apache.org> wrote: >>> >>>> >>>> >>>> On Wed, Feb 10, 2021 at 2:24 PM Alex Amato <ajam...@google.com> wrote: >>>> >>>>> >>>>> >>>>> On Wed, Feb 10, 2021 at 12:14 PM Kenneth Knowles <k...@apache.org> >>>>> wrote: >>>>> >>>>>> On a PR (https://github.com/apache/beam/pull/13927) we got into a >>>>>> discussion of a very old and strange feature of Beam that I think we >>>>>> should >>>>>> revisit. >>>>>> >>>>>> The WindowFn has the ability to shift timestamps forward in order to >>>>>> unblock downstream watermarks. Why? Specifically in this situation: >>>>>> >>>>>> - aggregation/GBK with overlapping windows like SlidingWindows >>>>>> - timestamp combiner of the aggregated outputs is EARLIEST of the >>>>>> inputs >>>>>> - there is another downstream aggregation/GBK >>>>>> >>>>>> The output watermark of the upstream aggregation is held to the >>>>>> minimum of the inputs. When an output is emitted, we desire the output to >>>>>> flow through the rest of the pipeline without delay. However, the >>>>>> downstream aggregation can (and often will) be delayed by the window size >>>>>> because of *watermark holds in other later windows that are not >>>>>> released until those windows output.* >>>>>> >>>>> Could you describe this a bit more? Why would later windows hold up >>>>> the watermark for upstream steps. (Is it due to some subtlety? Such as >>>>> tracking the watermark for each stage, rather than for each step?) >>>>> >>>> >>>> It does not have to do with stages/fusion (a runner-specific concept) >>>> but is a necessity of watermarks being per-PCollection. >>>> >>>> Suppose: >>>> >>>> - Default triggering >>>> - Timestamp combiner EARLIEST >>>> - 60s windows sliding every 10s >>>> - An element with timestamp 42 >>>> - Aggregation (A) with downstream aggregation (B) >>>> >>>> Here is what happens: >>>> >>>> - The element falls into [-10, 50) and [0, 60) and [10, 70) and [20, >>>> 80) and [30, 90) and [40, 100) >>>> - For each of those windows the output watermark hold is set to 42 >>>> (the element's timestamp) >>>> - At time 50 the aggregation (A) over the first window is emitted; the >>>> other windows remain buffered and held >>>> - The element arrives at aggregation (B) and is buffered because the >>>> input watermark (which is the held output watermark from A) is still 42, >>>> even though no other data will arrive for that window (WLOG if elements >>>> from other keys are shuffled in) >>>> - The input watermark for aggregation (B) does not advance past 42 >>>> until the [40, 100) window is fired and releases its watermark hold >>>> >>>> It is, indeed, subtle. To me, anyhow. I was wrong - it is not delayed >>>> by the window size, but by the difference in end-of-window timestamps to >>>> all assigned windows (window size minus slide?) >>>> >>>> So to avoid this, what actually happens in Java today is that the >>>> watermark hold, and output timestamp, is set not to 42 but altered to 50 to >>>> not overlap the prior window. Timestamp of 50 is very nonintuitive since >>>> you asked for the EARLIEST of input timestamps. EARLIEST combiner plays an >>>> important role in CoGBK based joins in SQL, where the iterables are >>>> re-exploded with timestamps that may be the minimum of input elements. This >>>> shifting may actually break SQL... >>>> >>>> This predated our switch away from "delta from watermark" late data >>>> dropping to "window expiry" data dropping. So maybe there is some new way >>>> to set a hold that does not make data late or droppable but still use the >>>> EARLIEST timestamp. That is my question, for which I have not figured out >>>> the answer. >>>> >>> >>> This is, indeed, a very tough question... >>> >>> I'd say this is generally a problem with EARLIEST and non-aligned >>> windows. E.g. for sessions, a long key can hold up the watermark for all >>> others. Here we "know" what the hold up is, and can adjust for it. But I >>> don't think doing this adjustment is the right thing. It would certainly >>> seem to mess up the timestamp of the outputs from a join. And it's possible >>> that the values get re-windowed in which case this element should get >>> joined with itself from a later window (which I'll admit is a bit odd, but >>> maybe a reflection that multiple-windowing, like multi-firing triggering, >>> is non-local). >>> >>> Logicaly, the reason we want [-10 50) window for B to fire shortly after >>> the input watermark for A passes 50 because no non-late data coming out of >>> A could influence it. In some sense, the "watermark" for the [-10, 50) >>> windows has indeed passed, but not that for later windows. I don't think >>> the beam model requires that we have a single watermark, just that we fire >>> triggers/timers once we have seen all the on-time data that we think we >>> could, and a runner could be smart about this. >>> >>> We may want to keep the ability to shift timestamps for WindowFns, but I >>> think we shouldn't be doing so for the default sliding windows. Correctness >>> (of output timestamps) over latency unless one asks otherwise. >>> >>> >>>> Kenn >>>> >>>> >>>>> >>>>>> To avoid this problem, element x in window w will have its timestamp >>>>>> shifted to not overlap with any earlier windows. It is a weird behavior. >>>>>> It >>>>>> fixes the watermark hold problem but introduces a strange output with a >>>>>> mysterious timestamp that is hard to justify. >>>>>> >>>>>> Any other ideas? >>>>>> >>>>>> Kenn >>>>>> >>>>>
